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Walkout basement exterior waterproofing

9 years ago

Hello everyone. We are starting to build a 1 1/2 story walkout in GA (CZ 3). The basement walls are cement block. For waterproofing, our GC has proposed to install a rubberized asphalt membrane on the exterior walls and an exterior French drain system. Is this a sufficient setup to prevent water getting into the basement? Thanks!

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Comments (8)

pprioroh
9 years ago
IMO no. It may work for awhile, but rubberized asphalt is not going to span any significant cracks that might form (esp with block). French drain and proper drainage are of course critical, but if you're going to finish and use a basement IMO it's worth spending the extra money on proper waterproofing including either a more advanced membrane externally or a dimpled membrane system.

As well, external insulation will keep basement walls warmer and less likely to condense moist air and less likely to be damp/mildewy. We built our basement 10 years ago using these principles and have had a completely dry basement that is just as nice as the rest of our home (our rear is walk-out on grade as well)

It's a LOT more expensive to go back and try and fix problems later, should they occur. and if you are going to finish the space, it gets even more expensive should problems arise.
dcmyles
Original Author
9 years ago
Thanks for the reply. I'm not sure about "more advanced membrane"; will have to discuss with GC. We also entertained the use of external insulation but not sure if it's code approved for our termite area.
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Look at Hydroment's Ultraset. It's a trowel-on membrane. An excellent adhesive, a moisture barrier, and a crack isolation barrier all in one. For the areas that will have RFH, you can use the Ultraset as an adhesive to set something like Cerazorb, a synthetic cork. Some folks do use 1/4" or 6mm thick true cork on the slab to act as a thermal break, just make sure it's not a low-density cork and that it's recommended for that purpose. There are RFH websites that sell a proper cork product. The RFH can then go over the Cerazorb/cork. OR you can use the Ultraset to adhere RPM mats (or equivalent) to the slab and install your RFH over that. RPM mats will act as a therma break, even though they have a negligible R-value. RPM mats are lugged mats, the lugs act as anchors for weaving RFH wire. While the RPM mat has little R-value, cork on the other hand has an R-value of about R3, so you'd get roughly R 0.75 from a roll of 1/4" cork. In the areas that won't get RFH, you can use Ultraset to adhere 1/4" cork/Cerazorb to the slab, then install a floating engineered flooring over the cork. 1/4" thick Wedi board can also be used instead of cork/Cerazorb, etc. All those are low-profile options. You'll get a bit of a thermal break but very little improvement in R-value. If you want more R-value, look at sheets of XPS. XPS comes in 4' by 8' sheets and is about R5 per inch. As you can see there are numerous ways to get this done. Some products have certain advantages over others.
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I found the same situation regarding conflicting data on foam and vapor permeance (both from building science sources). I believe the newer thinking is that concrete doesn't need to "dry" to the inside. It's naturally damp. It wicks. Accept it and move on. It's not really going to STAY dry unless you live in the desert southwest and then maybe not even then. Assuming you don't have flooding, put the tightest seal you can against that wall. Think: - How am I going to manage moisture and airflow so R-value doesn't matter as much? - How can I make sure water (yes, there is always water in concrete) doesn't make if through into the wall. I'm choosing the same components as hime but with thinner XPS since I'm in the south, below grade, and only really need it for vapor/thermal break. The R5 XPS sheets and the R15 Roxul batts are within about $40 for the same square footage, but they perform different roles, so I'm using both. I used blocks of XPS and can-foam in the rim joists to completely seal them off. It was a little slow, but not bad to do with some music and a beer. That one week worth of evenings made the biggest single difference in noise, leakage, and temp control in the basement (it's really the only part above grade). In a basement, fiberglass's few advantages (cost and cost) are vastly outweighed by the disadvantages (glass fiber, difficulty to get installed just right, breeds mold, holds moisture, doesn't block airflow). Don't think I'm just fearmongering... It's possible to use fiberglass in a place like this and be really meticulous and have it work fine. I've seen lots of friends and neighbors use it and don't have giant mold issues....yet. I'd rather spend a little more on mineral wool for the piece of mind. Permanence really doesn't vary that much between different thicknesses of XPS. Moreso in EPS since it lets moisture through pretty easily. As long as you have enough to keep the inside temp of the foam above the dewpoint for the coldest average climate days in your region (google-able on how to calculate that), you should be good on that. Just think of it as a system, focus on keeping the envelope tight and making sure there are no gaps where cool meets warm (or cool meets insulation, more accurately). Pay attention to the little details where dissimilar things meet each other. Those are where the problems happen.
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worthy
9 years ago
Delta MS or equivalent is sufficient in most soil conditions. That's what I have used on block walls, along with the mandatory dampproofing bituminous sprays. As noted above, rubberized sprays have a limited ability to bridge cracks. To aid in free draining to footing drains, I've also backfilled with sand and/or other freeflowing materials in addition to using a membrane.

Other waterproofing materials include peel and stick membranes, alternating layers of bituminious and fiberglass (an effective old-style approach) and crystalline waterproofing.

There are a number of niggling details with external insulation of the basement in the abovegrade portion. It has to be protected from mechanical damage and can't be continuous otherwise it can become a perfect hidden highway for termites.
dcmyles
Original Author
9 years ago
Worthy, thanks for the additional resources. Our code official said no to any external insulation that comes into contact with below grade soil because of termites. We'll figure something out. Thanks again!
pprioroh
9 years ago
I don't understand this termite issue - building science website recommends external insulation and that's what we did on our current home and basement walls are always warm.

Is the concern that termites will burrow behind the insulation and up into the structure itself? Our insulation stops several inches below the upper structure.

Or is it that the termites will actually live in the insulation itself?

Either way, if it's such a concern and against code, why does buildingscience have it as the recommended construction technique?
worthy
9 years ago
There are several problems with external foam insulation of the basement: difficulty in protecting the foam during construction; bridging when cladding the house with masonry (see illustration below); protection of the abovegrade portion of the foam; and the coverage it gives for termite highways into the wood structure of the house.

Though termites are often considered mostly a southern US problem, they are widespread in other climes too. Termites are endemic in parts of Toronto, for instance. Indeed, in my capacity as a mortgage manager, I usually require a termite inspection of older properties in certain neighbourhoods.

Source: Building Science Corp.
Mossonarock
9 years ago
Wow, thermal bridging with brick veneer. I learned something.
pprioroh
9 years ago
Anyone comment on the Horizon Foundation Finishing System (http://www.guaranteeddrybasements.com/products/default.asp?id=12 and http://www.bonedry.us/products_hor.asp)

It is basically (I think) a glass matte layer with an elastomeric "stucco" like spray over the top of the tuff-n-dry fiberglass insulation. I saw a "demo wall" at a recent builder who uses this as his foundation waterproofing and offers the above grade portion (the "horizon" part) as an upgrade.

It seemed VERY durable (when nobody was around I poked and prodded pretty aggressively and it wouldn't crack or break despite indenting the insulation underneath quite a bit) and looks very nice. Thinking about using this stuff all the way up to my siding on a walk out basement so have several feet exposed.

(no connection other than possible purchaser...)

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